TIMER-BASED THERMAL PROTECTION FOR POWER COMPONENTS Of A SWITCH MODE POWER SUPPLY

ABSTRACT

The maximum time that external components of a switch mode power supply over-conduct is determined by an actual ambient temperature at which the devices are operating before they are turned on. Their operation time is thus extended when temperatures are low and decreased when temperatures are high.

BACKGROUND

Switch mode power supplies are well known. They typically comprise orinclude an integrated circuit controller, inductors, capacitors and highcurrent-carrying semi-conductors, typically diodes and field effectivetransistors (FET), which are controlled by the integrated circuitcontroller to switch and dissipate large amounts of power. With theexception of so-called smart MOSFETs, the diodes and FETs commonly usedwith switch mode power supplies do not have thermal protection and aresusceptible to thermal damage.

A well-known feature of switch mode power supplies is their ability toprovide a relatively constant output voltage, even when an input voltageto the supply is low or falling. In “automotive” applications, switchmode power supplies are often required to operate when a vehicle'sstorage battery terminal voltage is very low, e.g., about three volts,as often happens during engine cranking. When the terminal voltage of astorage battery voltage in a vehicle goes low, a switch mode powersupply must then be able to carry correspondingly large amounts of inputcurrent in order to maintain its output voltage. Stated another way, inorder to maintain a constant output power, the average input current toa switch mode power supply may have to increase substantially, if onlyfor short periods of time.

It is also well known that switch mode power supply efficiency candecrease as the input voltage to the supply decreases. Efficiency lossesfurther increase the average input current required to provide aconstant output voltage.

Excessive power dissipation caused by excessive current flow through asemi-conductors of a switch mode power supply will cause the device tofail prematurely. If such devices do not have their own thermalprotection, they will eventually be destroyed.

Prior art switch mode power supplies employ three different techniquesto avoid destroying a semi-conductor by excessive heat dissipationcaused by current flow. One solution is to over-size the external powercomponents or over-design the heat sync for the devices such that thesemi-conductors can operate indefinitely under worst case conditions. Anobvious drawback of such a solution is the increased size and cost ofexternal power components beyond what is necessary and is often anunacceptable solution where cost and physical size of an electroniccircuit is important.

Another prior art solution to protecting devices from over temperatureis to provide semi-conductors with their own built-in thermalprotection. Many semi-conductor manufacturers offer so-called “smartMOSFETs” with built-in thermal shutdown mechanisms. These components aremore expensive than unprotected MOSFETs and for that reason, using themis often undesirable.

A third method of protecting semi-conductors from over temperaturedestruction is to limit the time that they operate at an excessive inputcurrent in order to limit their temperature rise. Operating parametersof the power supply are used to determine the power dissipation on theexternal power components above typical values. A timer is started thatdetermines the amount of time that the external components can be usedbefore the power supply is shut off. When the timer reaches itspredetermined maximum count, power to the semi-conductors is shut off,preventing them from destruction.

With regard to the third solution, a method and apparatus for optimizingor maximizing the time during which semi-conductors can be operatedwithout destruction would be an improvement over the prior art.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram of a switch mode power supply havingtimer-based thermal protection for the power components of the supply;and

FIG. 2 depicts steps of a method of protecting external power componentsof a switch mode power supply from thermal damage.

DETAILED DESCRIPTION

FIG. 1 depicts a switch mode power supply 100. The power supply 100comprises a “power stage” 102 typically comprised of high-currentcarrying inductors, diodes, field effect transistors (FETs) andcapacitors that receive electrical energy from a vehicle battery 104.The battery of course has a nominal output voltage and it provides thatvoltage to the input of the power supply 100, which converts it tovoltage 106 necessary to operate a load 108, typically an enginecontroller unit or ECU. The ECU controls an engine and other componentsof a motor vehicle.

The power stage FETs 102 are turned on and off by voltages applied totheir gate terminals. Such voltages are considered to be control signalsand in FIG. 1, they are received from a pulse width modulation (PWM)controller 110. The output signals 112 from the PWM controller 110 turnthe power stage transistors 102 on and off at a rate and duty cycle bywhich the output voltage 106 can be maintained relatively constant, evenwhile the battery voltage 104 is low or decreasing.

The battery voltage 104 is also provided to a conventional analogvoltage comparator 114. The comparator 114 compares the actual batteryvoltage 104 to a reference voltage 116 and, when the battery voltage 104drops below the reference voltage 116, the comparator's output 118controls (starts and stops) a timer or counter 120 operatively coupledto an oscillator 122 which provides timing pulses to the counter 120.

Put simply, the timer, or counter, 120 provides a measurement of anamount of time that elapses after the input signal 118 from thecomparator is asserted and received by the counter 120. The output 124of the counter 120 is a digital value that is provided to a digitalcomparator 126. The digital comparator 126, which can be embodied aseither a processor or combinational logic, compares the output 124 ofthe counter 120 to a maximum count value 128. When the counter value 124exceeds the maximum count value 128, the digital comparator 126 providesan “inhibit” signal 130 to a control input 132 of the controller 110.The signal 130 of the digital comparator 126 to the controller 110causes the controller 110 to provide an output signal 112 to thetransistors of the power stage 102, which causes the high currentcarrying-transistors to shut off, preventing them from overheating.

Stated another way, when the battery voltage 104 is below or less than areference voltage 116, a counter 120 is started. When the elapsed timeas measured by the counter 120 is equals or exceeds a maximum value,which is loaded into a maximum counter register 128, the transistors ofthe power stage 102 are shut off. The circuitry depicted in FIG. 1 thuslimits the amount of time that the transistors of the power stage 102are allowed to conduct when the battery voltage 104 goes below somepre-determined value loaded into the maximum count register 128.

An improved thermal protection circuit which maximizes the time that thetransistors of the power stage 102 can over-conduct is provided by anambient temperature sensor 140 that provides a digital output countvalue 142 to the maximum count register 128. The ambient temperaturesensor thus effectively determines the maximum amount of time that thetransistors of the power stage 102 can conduct current as a function oftemperature. A thermal protection circuit can thus be considered tocomprise the counter 120, the maximum count register 128, the digitalcomparator 126 and the ambient temperature sensor 140.

Providing a digital count value 142 to a maximum count register 128 thatcorresponds to a temperature can be accomplished a number ofconventional ways. By way of example, a voltage provided by aconventional temperature sensor, e.g., a PN junction voltage, can beconverted to a digital value by a conventional A/D converter. Thedigital value from an A/D converter can be provided directly to themaximum count register 128 or have its value offset or “adjusted” up ordown (increased or decreased) using conventional digital logic circuitryor a processor.

As used herein, the term “real time” refers to the actual time duringwhich something takes place.

FIG. 2 depicts steps of a method 200 for protecting the external powercomponents of a switch mode power supply from thermal damage. The method200 comprises a first step 201, which comprises determining, if thermaloverload of the power stage components is possible (for example, batteryvoltage level is below predetermined threshold). In case thermaloverload is not possible, the timer is decremented (or reset) and noaction is taken to shut off the components of the power stage 208. Incase thermal overload is possible, a timer is enabled 205, and anambient temperature value is obtained at step 202 in real time. Theambient temperature is used to generate or create a digital valuecorresponding to a maximum amount of time that the current-carrying FETsof a switch mode power supply can conduct current. In alternateembodiments, a loop counter can be incremented or decremented until avalue is reached with the elapsed time that the FETs are on beingeffectively determined by the rate or speed at which the loop counterruns.

At step 204, a determination is made of the maximum amount of time toallow over-current to flow through the power switching transistors ofthe switch mode power supply using a counter value obtained using theambient temperature. At step 206, current is allowed to flow through thetransistors until the maximum over-current time is exceeded or equaled.

Current is provided to the load at step 208 until the maximumover-current time is exceeded. In embodiments where a loop counter isused, current is provided until the loop counter terminates.

When the maximum over-current time is met or exceeded, or the loopcounter terminates, current to the load is shut off at step 210,preventing the external switch mode power supply transistors fromthermal damage, the amount of time being dependent upon the ambienttemperature at which the transistors are being operated.

The method, as well as the apparatus, described herein extends orreduces the time that over-current is provided to external transistorsof a switch mode power supply according to their actual operatingconditions, i.e., according to the ambient temperature at which theystart conducting current. When ambient temperatures are very low, thetime that the external switch mode power supply transistors can beoperated is extended. Conversely, when ambient temperatures are high,the switch mode transistors are disabled before they are damaged muchsooner.

Those of ordinary skill in the art should recognize that the fieldeffect transistors that comprise the power stage 102 are three-terminaldevices. The devices' drain terminal is typically connected to thebattery; the source terminal is typically connected to the load. Thegate, however, and which controls the FETs, is coupled to the output ofthe controller 110. The gate voltage applied to the FETs thus determineswhether the transistors are on and conductive or off and non-conductive.

As is well-known, the frequency and the duty cycle of the transistors ofa switch mode power supply enable the transistors of the power stage toprovide current to the load at a voltage that is substantiallyindependent of the battery voltage 104. The processor 110 thusdetermines the frequency and duty cycle of the signals provided to thecontrol inputs, i.e. the gate terminals, of the transistors of the powerstage 102.

In a preferred embodiment, the maximum count register 128 is embodied asa series of D-type flip flops. It can also be embodied as a location ina memory device.

In the preferred embodiment, the ambient temperature sensor 140 isembodied as a diode formed on the same semi-conductor die as the othercomponents shown in FIG. 1. A diode is well-known to have a P-N junctionvoltage dependent upon its temperature.

The foregoing description is for purposes of illustration only. The truescope of the invention is set forth in the following claims.

What is claimed is:
 1. A switch-mode power supply comprising: atransistor, configured to provide current to a load from a batteryhaving an output voltage, the output voltage from the battery changingresponsive to load applied to the battery through the transistor, thetransistor having an input node, which is coupled to the battery, anoutput node that is coupled to the load and a control node, whereinsignals applied to the control node determine whether the transistor ison and conductive or, off and non-conductive; a processor having acontrol input and an output coupled to the control node of thetransistor, the processor being configured to switch the transistor onand off and thereby control the transistor to provide current to theload, the output voltage from the transistor and provided to the loadbeing substantially independent of the input voltage to the transistorfrom the battery but dependent on the amount of current flowing throughthe transistor from the battery, the processor additionally configuredto disable the transistor responsive to a disable signal received by theprocessor at the processor's control input; a thermal protection circuitcoupled to the processor's control input, the thermal protection circuitcomprising an ambient temperature sensor, counter and comparator, thethermal protection circuit being configured to provide a disable signalto the processor responsive to: an ambient temperature as determined bythe ambient temperature sensor; the battery voltage; and the amount oftime that the transistor is on and providing current to the load.
 2. Theswitch mode power supply of claim 1, wherein the thermal protectioncircuit is configured to change the time after which the disable signalis provided to the processor responsive to changes in the ambienttemperature.
 3. The switch mode power supply of claim 1, wherein thecounter is a loop counter.
 4. The switch mode power supply of claim 1,wherein the counter is a timer.
 5. The switch mode power supply of claim1, wherein the thermal protection circuit comprises: a counter; amaximum count register; and a comparator operatively coupled to thecounter and the maximum count register, the comparator having an outputterminal, which is configured to provide the disable signal to theprocessor; wherein the maximum count register holds a count value, whichis determined by the ambient temperature.
 6. The switch mode powersupply of claim 1, wherein the thermal protection circuit comprises aloop counter and wherein the thermal protection circuit is configured tolimit the amount of time that the transistor is on by a count limitdetermined by the loop counter.
 7. The switch mode power supply of claim1, wherein the processor is formed on a semiconductor die, and whereinthe thermal protection circuit comprises a semiconductor device that isalso formed on the semiconductor die.
 8. A method of protecting externalpower components of a switch mode power supply from thermal damage, themethod comprising: determining an ambient temperature in substantiallyreal time; determining a maximum length of time to permit current toflow through the external power components using the ambienttemperature; providing current to a load through the external powercomponents and, starting a timer at substantially the same time thatcurrent is provided to the load; comparing elapsed time as measured bythe timer to the maximum length of time determined from the ambienttemperature; and shutting off current to the load through the externalpower components when the elapsed time equals or exceeds the maximumlength of time determined from the ambient temperature.
 9. The method ofclaim 5, wherein the step of determining a maximum length of time topermit current to flow through the external power components using theambient temperature comprises obtaining a maximum length of time from atable of time values, which are indexed in the table by ambienttemperature values.
 10. The method of claim 5, wherein the step ofdetermining a maximum length of time to permit current to flow throughthe external power components using the ambient temperature comprises,evaluating an equation that correlates a maximum length of time toambient temperatures.
 11. The method of claim 5, wherein the step ofdetermining a maximum length of time to permit current to flow throughthe external power components using the ambient temperature comprisesdetermining a maximum length of time using a loop counter.